New hollow fiber bioreactors for aerobic culture were introduced and Aspergillus niger for citric acid production was cultivated as a model system. These reactors consisted of a bundle mixed of hydrophilic membranes for liquid nutrient transport and hydrophobic membranes for gaseous nutrient transport. The cells were successfully cultivated. However, the polymeric hollow fiber membranes were compressed and blocked by excessive fungal cell growth. Citric acid was produced with a high volumetric productivity compared with that of shake-flask fermentation, but the long-term operation was not successful due to a rapid decrease of the production rate.